Force sensors of the vibration type are well known to those involved in the measurement of (weights). Force (weight) transducers of the vibration type are advantageous in that construction is simple and does not require use of an analog-to-digital converter because a digital value, the number of vibration waves, is directly produced. In a vibration type apparatus, a vibratory beam is excited, and vibrates at particular frequencies related to the amount of stress applied to the vibratory beam. The frequency of vibration is also dependant on the stiffness of the beam, which should remain relatively constant for a vibratory beam of a given length and cross-section (aspect ratio).
The mechanical Q of an apparatus including a vibratory beam as the force (weight) sensor is proportional to the ratio of the energy stored in the beam to the energy lost by the beam for each cycle of vibration. A system with a low Q is undesirable because damping of the vibration used to measure the weight will occur, resulting in a far less stable resonant frequency and an increased tendency to crossover to unwanted resonant frequencies. A system with a high Q will maintain the oscillations of the vibratory beam, can use a smaller source of external energy to excite the vibratory beam, and will possess a more stable resonant frequency.
When a vibratory beam is used as the force sensor in a weighing apparatus, a stress due to the weight being measured is applied to a first end of the beam, while the sensor is stably mounted at the second end of the beam. When a single vibratory beam is used as the sensor, however, vibratory energy is lost at the mounted end of the beam, resulting in a lower Q for the system and damping of the vibrations. With a single vibratory beam, there is no balancing of forces at the mounted end of the sensor. The single beam vibrates and applies a moment to the sensor at its mounted end. In order to avoid a loss of energy due to damping at the mounted end of the sensor, a pair of parallel vibratory beams forming a double-ended tuning fork can be provided as the sensor. Another method used to minimize the energy loss (and accompanying decrease in Q) resulting from the force tending to rotate the mounted end of a single vibratory beam involves attaching each end of the single beam to a heavy intermediate mass having a large inertia, which is connected to the rest of the apparatus using flexible members. This method, however, cannot completely cancel out the forces applied to and the energy lost at the mounted end, and increases the expense, size, and complexity of the weighing apparatus.
In a sensor of the double-ended tuning fork type, typically one piezoelectric element on one beam is used to excite the tuning fork, while a second piezoelectric element on the second vibratory beam is used as a vibration pickup element. The vibratory beams are coupled together at their respective first and second ends. The pair of vibratory beams will oscillate at a measurement frequency that is determined by the length, cross-section, and stiffness of the two beams, and by the stress applied to the two beams when a force (weight) is being measured. When each of the pair of vibratory beams is practically identical, they will both oscillate at the same measurement frequency, but will oscillate 180.degree. out of phase. As a result, at the ends of each beam, the vibrations from each beam will cancel each other out, thereby preventing any moment from being applied to the mounted end of the sensor. Therefore, less vibration energy of the sensor is lost at the ends of the beams, and force (weight) measuring apparatus using a pair of parallel vibratory beams will have a higher Q than a similar system with a single vibratory beam.
However, in a conventional weighing apparatus using a vibratory beam, it is extremely difficult to manufacture a sensor that has a high Q. Thus, for a sensor utilizing a pair of parallel vibratory beams, tight tolerances are required during manufacture to ensure that there is no mismatch between the two beams that will create a difference in the resonant frequencies of each beam. In particular, the manufacturer must ensure that the two beams are equal in length, cross-section, and stiffness, and the stress due to the force (weight) being measured must be applied equally to the first end of each of the two beams. Otherwise, the frequency difference decreases the Q and eventually causes bistable operations of the beams in an oscillator circuit. In some cases, the oscillation ceases. Because tight tolerances (within microns) are necessary, the vibratory beams are fabricated using a precise method of cutting. As a result, conventional vibration type weighing apparatus are not, for example, molded or fabricated using a press tool.
In practice, sensors constructed by molding or using a press tool achieve a Q only on the order of approximately 150-250. Therefore, there is a need for a force sensor having a vibratory beam or beams that can attain higher values of Q but which can be manufactured within a broader tolerance with lower cost methods, such as by molding or using a press tool.
Additionally, typical force sensors, such as disclosed in U.S. Pat. No. 4,215,570, disclose a double-ended tuning fork formed out of piezoelectric quartz. Several disadvantages are associated with these types of sensors. Correct crystallographic orientation of the sensor is required in order to minimize any dependence of the resonant frequencies of the sensor on temperature. The sensors are manufactured by photolithographic etching or diamond machining and are relatively expensive to produce. Furthermore, the quartz sensors are very delicate and cannot withstand a high loading. In practice, they are used to sense weights of only a few kilograms. When heavy weights are measured, the associated force is not directly applied to the fragile quartz sensor. Instead, a strain proportional to the weight is applied to the sensor by means of a lever arrangement. Therefore, such sensors must contain several additional parts, further increasing the cost of producing the apparatus.